1. Field of the invention
This invention relates to a novel process for the preparation of porous carbon plates. More specifically, it relates to a process for preparing a thick porous carbon plate excellent in chemical resistance, electric conductivity and strength by forming a sheet according to a paper-making method, impregnating the sheet with a polymeric material and carbonizing the impregnated sheet.
2. Description of the Prior Art
As the conventional process for preparing a carbon fiber sheet, there is known a process in which a carbon fiber-incorporated paper sheet is formed from a pulp, a binder and a carbon fiber according to a paper-making process. However, this carbon fiber-incorporated paper sheet has a relatively high electric resistance and is poor in the chemical resistance. Therefore, this paper sheet is not suitably used for an electrode of a fuel cell or the like. There also is known a process in which the above-mentioned carbon fiber-incorporated paper sheet is impregnated with a solution of a thermosetting resin and is heated to be thereby carbonized in an inert atmosphere again (see U.S. Pat. No. 3,998,689). Since an organic substance such as a pulp is carbonized by the heat treatment, according to this process, there can be obtained a fiber paper having a lower electric resistance and a better chemical resistance than that of the above-mentioned carbon fiber-incorporated paper sheet. However, since the carbon fiber per se has a high modulus of elasticity, the fiber contact portion is not sufficiently bonded. Accordingly, it is difficult to prepare a carbon fiber paper sheet having a sufficiently low electric resistance. Moreover, since the carbon fiber has a high specific gravity and a hot water-soluble binder is used, it is difficult to prepare a bulky porous plate and also to control the bulk density and pore size to levels suitable for various uses. Moreover, since the baking step must be conducted two times, the product is expensive, and reduction of the manufacturing cost is desired. Still further, according to the above-mentioned process, it is difficult to obtain a thick sheet having a uniform thickness, and since the carbon fiber has no hydrophilic property and a high modulus of elasticity, the adhesion to the pulp is poor and the sheet strength is low. Therefore, it is necessary to incorporate a binder fiber together with the pulp.
As means for increasing the strength of a carbon plate, there is ordinarily adopted a method in which a starting sheet is reinforced by an impregnation treatment with a solution of a phenolic resin or the like. However, in case of a thick sheet, the impregnation often becomes uneven and because of limitations of the impregnation and drying capacities, the productivity cannot be increased.